Final answer:
The nurse should have amiodarone available for immediate use as a medication to counteract the client's new, multiform premature ventricular contractions (PVCs) if they are allergic to lidocaine hydrochloride.
Step-by-step explanation:
If a client with myocardial infarction is experiencing new, multiform premature ventricular contractions (PVCs) and is allergic to lidocaine hydrochloride, the nurse should have amiodarone available for immediate use. Amiodarone is a potassium channel blocker, which is commonly used to treat abnormal electrical activity in the heart, such as cardiac dysrhythmia. It works by impeding the movement of potassium ions through voltage-gated potassium channels, helping to regulate the heart's rhythm.